Precise mass determination is important in modern mass spectrometry, particularly in biological mass spectrometry. No limit for the mass accuracy is known beyond which no further increase in the useful information content may be expected. Increasing the mass accuracy is therefore a goal which will continue to be pursued. A high mass accuracy alone is often not sufficient to solve a given analytical task, however. In addition to high mass accuracy, a high mass resolving power is particularly important because in biological mass spectrometry, in particular, ion signals with slight mass differences must frequently be detected and measured separately. In enzymatic digestion of protein mixtures, for example, there are thousands of ions in a mass spectrum; five to ten or more different ionic species of the same nominal mass number must often be separated and precisely measured. Crude oil mixtures even contain hundreds of ionic species with the same nominal mass number. The highest mass resolutions are nowadays achieved with Fourier transform mass spectrometers.
“Fourier transform mass spectrometers” (FT-MS) is the term used for all types of mass spectrometer in which ions of the same mass flying coherently in clouds that are oscillating, orbiting on circular trajectories or otherwise periodically moving, generate image currents in detection electrodes. These currents are stored as “transients” after being amplified and digitized; the frequencies of the periodic motions can be derived from these transients by Fourier analysis. The Fourier analysis transforms the sequence of the original image current measurements of the transient from a “time domain” into a sequence of frequency values in a “frequency domain”. The frequency signals of the different ionic species, which can be recognized as peaks in the frequency domain, can then be used to determine the mass-to charge ratios m/z and their intensities very precisely. There are several types of such Fourier transform mass spectrometer that will be briefly explained here.
In ion cyclotron resonance mass spectrometers (FT-ICR-MS), the mass-to-charge ratios m/z of the ions are measured by the frequencies of the orbital motions of clouds of coherently flying ions in strong magnetic fields. This is done in ICR measuring cells that are in a homogeneous magnetic field of high field strength. The ions, which are first introduced on the axis of the measuring cell and trapped there, are brought to the desired orbits by excitation of their cyclotron motions. The orbital motion normally includes superpositions of cyclotron and magnetron motions, with the magnetron motions slightly distorting the measurement of the cyclotron frequencies. The magnetic field is generated by superconducting magnet coils cooled with liquid helium. Nowadays, commercial mass spectrometers provide usable ICR measuring cell diameters of up to approximately 6 centimeters at magnetic field strengths of 7 to 18 tesla. Higher field strengths offer advantages, in that some of the quality factors for the mass spectrometers depend linearly on the field strength, and others even on the square of the field strength.
In the ICR measuring cells, the orbital frequency of the ions is measured in the most homogeneous part of the magnetic field. Measuring cells in the form of a cylindrical sheath are usually used. Such an ICR measuring cell is shown in FIG. 1. The ICR measuring cells usually comprise four longitudinal electrodes, e.g., 17, 18, 19, which extend parallel to the magnetic field lines and surround the inside of the measuring cell like a sheath. To prevent the ions escaping, trapping plates 16, whose potential keeps the ions in the cell, are mounted at the ends of the measuring cell. Two opposing longitudinal electrodes, 17 and 19 for example, are used to bring the ions introduced close to the axis through the trapping plates 16 to larger orbits of their cyclotron motion. Ions with the same mass-to-charge ratio m/z are excited as coherently as possible in order to obtain a cloud of ions orbiting in phase. The other two electrodes, of which only one 18 is visible here, serve to measure the orbiting of the ion clouds by their image currents, which are induced in the electrodes as the ion clouds fly past. Introducing the ions into the measuring cell, ion excitation and ion detection are carried out in successive phases of the method, as is known to anyone skilled in the art.
Since the mass-to-charge ratio of the ions is unknown before the measurement, they are excited by the longitudinal electrodes 17, 19, using a mixture of excitation frequencies which is as homogeneous as possible. This mixture can be a temporal mixture with frequencies increasing with time (this is then called a “chirp”), or it can be a synchronous computer-calculated mixture of all frequencies (a “sync pulse”); chirps are usually used.
The FT-ICR mass spectrometers are currently the most accurate of all types of mass spectrometer. The accuracy of the mass determination ultimately depends on the number of ion orbits that can be detected by the measurement, i.e., on the usable duration of the transient. Conventional measuring cells with four longitudinal electrodes and trapping electrodes at the ends provide image current transients with durations of up to a few seconds (usually up to around five seconds), which result in a resolution of around R=100,000 for ions of the mass-to-charge ratio m/z=1000 u (atomic mass units).
German Patent DE 10 2009 050 039.1 to I. V. Boldin and E. Nikolaev discloses an ICR measuring cell illustrated in FIG. 2 which establishes a new generation of high-performance ICR mass spectrometers. The measuring cell represents the latest state of the art for the ICR measuring technology; it has a cylindrical sheath which is divided by parabolic separating gaps into crown, diamond and lancet-shaped sheath electrodes segments 60 to 64. The measuring cell surprisingly provides resolutions far in excess of one million for ions of mass m/z=1000 u, even in moderately strong magnetic fields of only seven tesla when complex mixtures are present, and far in excess of ten million for isolated ionic species. As simulations in supercomputers have shown, the measuring cell has coherence-focusing characteristics: the clouds of the individual ionic species are each held close together, so transients with a duration of several minutes can be measured. There is still no simple, intuitive explanation for the mechanism of coherence focusing, but it can be assumed that it is connected with the many slight potential jumps which the ions experience on their trajectory.
Although ICR mass spectrometers are quite outstanding, they still have the disadvantage that they must be operated with superconducting magnets. They are therefore expensive, heavy and unwieldy to handle. For a number of years now, electrostatic Fourier transform mass spectrometers have been successfully marketed in competition with ICR mass spectrometers; they provide a similarly high resolution but are much smaller.
This second type of Fourier transform mass spectrometer is based on Kingdon ion traps. Kingdon ion traps are generally electrostatic ion traps in which ions can orbit one or more inner electrodes or oscillate through between several inner electrodes, without there being any magnetic field. An outer, enclosing housing is at a DC potential which the ions with a set kinetic energy cannot reach. In special Kingdon ion traps suitable as measuring cells for mass spectrometers, the interior surfaces of the housing electrodes and the outer surfaces of the inner electrodes are designed so that, firstly, the motions of the ions in the longitudinal direction of the Kingdon ion trap are completely decoupled from their motions in the transverse direction and, secondly, a parabolic potential well is generated in the longitudinal direction in which the ions can oscillate harmonically. Here, the term “Kingdon ion trap”, and especially the term “Kingdon measuring cell”, refers only to these special forms in which ions can oscillate harmonically in the longitudinal direction, completely decoupled from their motions in the transverse direction.
If clouds of coherently flying ions move longitudinally in the parabolic potential profile, the ion clouds with different charge-related masses each oscillate with their own, mass-dependent frequencies. The frequencies are inversely proportional to the square root √(m/z) of the charge-related mass m/z. The two electrodes of a housing with a central, transverse split, for example, are suitable as detection electrodes for image current measurements. The oscillating ions induce image currents that can be stored as transients. A Fourier analysis can be used to obtain a frequency spectrum from these transients, as has already been described above, and the mass spectrum can then be obtained from this by conversion.
U.S. Pat. No. 5,886,346 to A. A. Makarov discusses the fundamentals of a special Kingdon ion trap which was launched by Thermo-Fischer Scientific GmbH Bremen under the name Orbitrap®. FIG. 3 represents such an electrostatic ion trap. The decoupling of the motions in the transverse and axial direction is achieved solely by the special shape of the electrodes. The Orbitrap® trap consists of a single spindle-shaped inner electrode 13 and coaxial housing electrodes 11, 12 transversely split down the center. The housing electrodes have an ion-repelling electric potential, and the inner electrode an ion-attracting electric potential. With the aid of an ion lens, the ions are tangentially injected as ion packets through an opening in the housing electrode, and they circulate on orbital and axial trajectories 14 in a hyper-logarithmic electric potential. The kinetic injection energy of the ions is adjusted so that the attractive forces and the centrifugal forces of the orbital motion cancel each other out, and the ions therefore largely move on virtually circular trajectories. The maximum useful duration of the image current transients of an Orbitrap® trap is (similar to conventional ICR mass spectrometers) in the order of around five seconds. The mass resolution is currently around R=100,000 at m/z=1,000 atomic mass units; with good instruments it can be higher.
German Patent DE 10 2007 024 858 A1 to C. Köster discloses additional types of Kingdon ion traps which have several inner electrodes. These Kingdon measuring cells can be produced with the same decoupling of the ions' radial and axial motion. The ions can oscillate in a plane between two inner electrodes, for example, which produces a particularly simple way of introducing the ions into a Kingdon measuring cell.
An advantage of Kingdon ion trap mass spectrometers compared to ion cyclotron resonance mass spectrometers (ICR-MS) with similarly high mass resolutions R is that no magnet is required for storing the ions, and so the technical set-up is much less complex. Even bench-top instruments are conceivable. The ions are stored here either oscillating or orbiting in a DC field, and thus require only DC voltages at the electrodes, but these DC voltages must be kept constant with a very high degree of precision. Moreover, the decrease in resolution R towards higher ion masses in Kingdon ion trap mass spectrometers is only inversely proportional to the square root √(m/z) of the mass-to-charge ratio m/z of the ions, whereas in ICR-MS the decrease in resolution R is inversely proportional to the charge-related mass m/z itself; this means the resolution falls off much more rapidly toward higher masses in ICR-MS in an unfavorable way.
It is not yet known why the useful duration of the image current transient in Kingdon measuring cells is limited to an order of magnitude of around five seconds. Very good ultrahigh vacua, of better than 10−7 pascal if possible, must be generated in Kingdon measuring cells (as is the case in ICR measuring cells) in order for collisions not to force the ions from their trajectory. The mean free path of the ions must amount to hundreds of kilometers. The limitation of the image current transient may therefore be attributable to a residual pressure in the almost closed measuring cells, which are very difficult to evacuate. On the other hand, it is possible that slight flaws in the shape of the inner and outer electrodes, which have to be manufactured with highest precision, limit the useful duration of the image current transient. Deviations in shape can generate a tiny residual coupling of the axial and transverse ion motions, especially in conjunction with angular and energy variations of the ion injection. Even a very weak residual coupling may have devastating effects on the ion trajectories after the ions have orbited a few ten thousand times. As is known from coupled oscillation systems, there are necessarily transitions of the energy from one direction of oscillation to the other, which means, for example, that the axial oscillation amplitude can increase so much that the ions impact on the outer electrodes and are thus destroyed. The Kingdon measuring cells described here decouple the axial and transverse ion motions solely by their shape; there is no mechanical or electrical correction when the device is in operation. Particularly, there is no attempt at a coherence focusing of any kind which may counteract a residual coupling.
The hyperlogarithmic electric field also can be generated by completely other forms of cells. A very simple possibility includes dividing the surfaces of both an inner and an outer cylinder, as is shown in FIG. 4, into electrode rings, which are insulated from each other, and applying potentials, which increase parabolically from the center outward to the ends so that in the space between the cylindrical surfaces an essentially parabolic potential well is created along the axis for the ions introduced. This requires at least five, but preferably a much larger number of ring electrodes per cylindrical sheath. An identical voltage difference is applied between corresponding rings of the inner and the outer cylindrical sheath so that a radial field which is practically constant over the length is generated between the cylindrical sheaths, and ions with appropriate kinetic energy can orbit around the inner cylinder in this radial field. Such cylindrical Kingdon ion traps are described in published PCT Application WO 2007/000587 to A. A. Makarov and U.S. Published Patent Application 2009/0078866 A1 to G. Li and A. Mordehai.
When the term “acquisition of a mass spectrum” or a similar phrase is used below in connection with Fourier transform mass spectrometers, this includes the entire sequence of steps from the filling of the measuring cell with ions, excitation of the ions to cyclotron orbits or oscillations, measurement of the image current transients, digitization, Fourier transform, determination of the frequencies of the individual ionic species and, finally, calculation of the mass-to-charge ratios and intensities of the ionic species which represent the mass spectrum.
In view of the above there is a need of providing a measuring device with an electrostatic measuring cell for measuring ion oscillations in potential wells; this measuring cell, in particular, being easier and more efficient to evacuate than current electrostatic measuring cells, allowing field corrections for the decoupling of the axial and transverse motions of the ions when the device is in operation, and even providing coherence focusing if possible.